Port Scanning with Nmap

Previously, we learned about the three-way handshake, ports, and protocols. Now let’s look at how Nmap finds open ports in a system.

Identifying ports

Nmap crafts custom packets of data and sends them to the server that’s being scanned. Based on the response (or lack of it), Nmap classifies ports as open or closed. For example, if Nmap sends an SYN request to port 22 and gets an RST flag, Nmap determines that the port is closed. If Nmap gets a SYN-ACK response back from the server, it means that the port is open and ready to accept incoming connections.

Once Nmap gets a SYN-ACK response back, we can either choose to drop the connection or continue the connection to gather more ...